The present invention relates to an automatic balancing device for maintaining rotational balance, a rotational driving device including the automatic balancing device, and a disk drive including the rotational driving device.
In recent disk drives for recording/reproducing data, such as an optical disk drive and a magnetic disk drive, there is a case in which when a disk placed on a turntable is rotated, the rotational balance of the disk may be lost and cause a reduction in the stability of recording/reproduction.
As a technique for improving the rotational balance of the disk, there has been disclosed a technique such that a ring-like member having a hollow portion for accommodating a magnetic fluid is provided on a rotating shaft of a motor so as to be rotatable integrally with the rotating shaft. The ring-like member has a central boss portion, and a ring magnet is provided on the outer circumferential surface of the boss portion. The motor is supported to a subchassis, which is supported through an elastic member to a main chassis. With this structure, magnetic fluid is magnetically attracted to the ring magnet at low rotational speeds of the motor, thereby maintaining the rotational balance. When the rotational speed of the motor increases and a centrifugal force due to the rotation of the ring-like member accordingly increases accordingly, the magnetic fluid is moved toward the outer circumference of the ring-like member, for example, so as to maintain the rotational balance (see Japanese Patent Laid-open No. 4-312244, Column [0006] and FIG. 1, for example).
In the related art cited above, however, the distance between the ring magnet and the magnetic fluid staying at a lower portion of the ring-like member is large when the disk drive is used in its vertical position, i.e., in the condition where the recording surface of the disk is perpendicular to a horizontal plane. Accordingly, it is difficult to return and retain the magnetic fluid to the ring magnet in such a vertical position. As a result, there arises a problem that it is difficult to maintain the rotational balance of the disk, depending upon the position of the disk. Further, there arises another problem that the magnetic fluid is moved by the centrifugal force in a direction substantially opposite to the direction of deviation of the center of rotation of the disk, causing a flow of the magnetic fluid along the outer circumference of the ring-like member. As a result, the deviation of the magnetic fluid is lost and it is accordingly difficult to greatly improve the rotational balance with a small amplitude of oscillation.